This invention relates to a supercalender to be installed at the output of a paper producing machine in an "in-line" arrangement.
Paper appearing at the output of a paper machine has, in a raw condition, a relatively rough surface and requires for most applications further processing which results in a leveling and a compression of the surface. The additional processing is generally accomplished by calenders and supercalenders. Calenders comprise only hard rollers and level the peripheral surfaces of the paper so that the parts of the paper forming the outer surface lie essentially in one plane.
The roller gaps or nips of a supercalender are so called soft roller gaps, i.e., those in which a hard roller is paired with an elastically yielding roller. Elastic rollers in supercalenders are usually in the form of paper rollers, i.e., drums made of layers of paper sheets. Calenders compress the paper web and close pores in the web's outer surface, thereby smoothing and lending luster to the paper.
Supercalenders generally comprise a vertically arrayed stack of rollers which are alternately hard and soft. In such a supercalender the paper web runs through a succession of roller nips.
The currently predominant practice involves the processing of the paper surface at a separate work station downstream of, i.e., at the output of, the paper producing machine. The paper web is rolled up at the output of the paper machine, subsequently unwound in a separate work stage, surface processed and then again rolled up. In most cases two supercalenders are necessary to handle the production of a paper machine, which at elevated working speeds can produce up to 1000 meters of paper per minute, because disturbances during processing of the paper surface frequently result in a shut down of the operating supercalender. A single supercalender is thus insufficient to keep up with the production of a fast paper machine.
The separate processing of the paper surface in a so-called "off line" assembly is disadvantageous owing to the necessity of additional winding stages. The ideal circumstance is to have the paper pass through the surface processing stations in a continuous train at the output of the paper producing machine, the paper being continuously produced in its final state at the end of the processing steps.
It has been known for a long time to integrate calenders into the paper machines for smoothing the surface of the paper web. The effects achieved with calenders are, however, satisfactory only for certain applications and are undesirable for other applications, for example, because dark spots can arise in the paper surface owing to locally varying compression of the paper fibers.
The integration of supercalenders into paper machines has been undertaken to date only in isolated cases for special purposes. In addition to the difficulties the entire installation would suffer upon a breakdown or malfunction in a supercalender integrated into the paper machine, there is the problem that the yielding or flexible rollers would not be able to withstand the stresses which are produced at high speeds in the faster paper machines.
An example of a supercalender integrated into a paper machine is described in the German language magazine "Wochenblatt fur Papierfabrication" ("Weekly Paper for Paper Making"), Volume 21, 1978, pages 809-814, and in particular page 814. This so-called glazing supercalender comprises a stack of five rollers, of which three are hard and two soft. The paper web passes through four roller nips. In order to achieve the desired effect, relatively high pressures are required. The use of such pressures has frequently resulted in the deformation into polygonal shapes of the outer layers of relatively soft rollers, which deformation leads, during machine operation, to vibrations which are difficult to control. Moreover, in the familiar form of implementation it is problematic that the soft rollers each function at two roller nips and thus experience two milling operations during each rotation. The concomitant transfer of milling work into heat leads to an increase in temperature of the coverings of the elastic rollers, and the existence of a maximum allowable temperature means a limit on the working speed of the supercalender. The familiar glazing calendar has a maximum speed of 250 m/min.
This disadvantage is not present in the supercalender described in European patent application (EUOS) No. 27,621 published Apr. 29, 1981 "in line" utilization. This supercalender comprises at least two cofunctioning hard rollers, against which a total of four soft rollers press from the side. The paper web meanders through the roller assembly and alternately traverses soft roller nips and hard roller nips. Further hard rollers can be placed from the outside against the first two rollers in the same plane, so that the number of the hard roller nips can be increased. The soft rollers are supposed to be paper rollers, i.e., made of paper layers, whereby the calendar is suitable only for paper machines of low speed. In addition, because the rollers do not lie in a single plane, adjusting the rollers is very expensive and stable operation of the assembly is not easy.
An object of the present invention is to provide a supercalender for in-line operation, which is of simple construction and which enables the use of higher speeds than possible with conventional supercalenders.